1. Field of the Invention
The present invention relates to an optical sensor having a multistage type thermoelectric cooling section.
2. Related Background Art
For example, in a CCD image sensor or the like, a number of photosensors, such as photodiodes, for example, are arranged in parallel as a photosensitive section, whereas the photosensors are provided with a thermoelectric cooling section for cooling the photosensors. As the thermoelectric cooling section, there has been known the one in which Peltier devices are stacked on each other in a plurality of stages for effecting forcible air cooling.
In the thermoelectric cooling section in which Peltier devices are stacked on each other in a plurality of stages, it is necessary that, in order to improve the heat radiation efficiency, a Peltier device having a higher output (greater current consumption) than that of a Peltier device on the heat absorption side nearer the photosensors is disposed on the heat radiation side. Consequently, Peltier devices having different output characteristics [I (current)-V (voltage) characteristics] are disposed on the heat absorption side and the heat radiation side, respectively.
When the Peltier devices having different output characteristics are thus stacked on each other, it is necessary for each Peltier device to operate at its optimum operating point, i.e., the point where the cooling performance of the Peltier device is maximized with respect to its consumed power, in order to exhibit the characteristics of the individual Peltier devices at maximum.
For this reason, the individual Peltier devices have conventionally been driven at their optimum operating points by their respective power sources independent from each other.